The present invention relates to a method for cooling the gas flow exiting through the uptake in a smelting furnace such as a flash-smelting furnace with finely dispersed liquid sprays, which prevent the solid matter particles contained within from coalescing and sintering.
As is well known, a flash-smelting furnace consists of a reaction shaft, lower furnace and uptake. The concentrate to be smelted and admixtures are fed into the reaction shaft, where they melt and settle to the lower furnace, forming at least matte and slag layers from the concentrate and flux. The exhaust gases which formed and reacted during the handling, are removed in the uptake, where also the finest part of the melt droplets tend to remove as flue dust along with the gases. Gases are usually cooled and scrubbed in a waste heat boiler and electrostatic precipitator placed after the uptake. The purpose of the radiation section of the waste heat boiler is to cool the gases, so that the melt particles solidify and the temperature drops to below the sintering temperature of the particles. The gases exiting the smelting furnace can also be treated in a different way.
The temperature of the gases leaving via the uptake of a flash-smelting furnace is of the order of 1300xc2x0 C., depending to a certain extent on the material being smelted. Often, the melt droplets that flow together with the exhaust gases tend to sinter together and attach to the walls, and for instance to the outlet leading from the uptake to the after-treatment equipment, the so-called neck of the uptake. The removal of build-ups from the neck is only possible by chiselling or shooting, which cause disruption in production.
In certain cases, e.g. the flash-smelting process for nickel, the flue particles flowing with the exhaust gases are oxidic, usually Fe or Ni. In this case, the melting point of the flue dust generated in the flash-smelting furnace can rise to such a high point, that it is not feasible to keep the flue particles molten outside the,uptake, although the temperature of said uptake can be raised to some extent by burning additional fuel there. This results in build-ups caused by the dust particles particularly at the gas outlet point.
A method has now been developed to overcome the previously described disadvantage, namely removing of the build-ups formed by dust particles and the method reduces formation of said build-ups and alters the nature of the build-up. According to the method developed, the temperature of the exhaust gas flow and dust particles contained therein coming from the smelting furnace is lowered at the uptake stage by a finely dispersed liquid sprays, so that the particles neither sinter not attach themselves to surfaces but instead, they travel together with the gas flow out of the furnace. The dust, which settles in the neck, is so brittle and fine grained, that it can be easily blown from the outlet without interrupting the process. The essential features of this invention will become apparent in the attached patent claims.
In practice, the cooling of dust occurs by spraying very fine water droplets into the uptake, for example, via radial nozzles from the walls of the uptake to inside the furnace. The amount of water to be sprayed should be small enough that it evaporates entirely, so that not even a single water drop could cause any risk of explosion on the surface of the melt. It has been proved, that a sufficient temperature drop in the dust particles can be achieved when the amount of water to be, sprayed is 40-120 kg/1000 Nm3 of the gas flow in the uptake shaft.